: Recent bombesin and gastrin releasing peptide receptor antagonists have been demonstrated to be quite potent in inhibiting the effects of bombesin in pancreatic acinar cells, brainmembranes, Swiss 3T3 fibroblasts, and human small cell lung carcinoma (SCLC). However, these antagonists do not display comparable high potency in inhibiting the proliferation of human small cell lung carcinoma. Certain antagonist analogues do inhibit proliferation of clonal growth of SCLC and have been found to inhibit SCLC tumor growth in vivo. Studies will be conducted to design and synthesize more stable compounds from the inhibitor structures by cyclization and decreasing their susceptibility to proteolytic degradation. Novel groups will be added to the analogues to increase interaction with the SCLC receptor. The effect on clonal growth of human SCLC cells will be measured as an indicator of in vivo anti-tumor potential.